Add Error message in PHP

[ghjk]

I`m using php and mysql. I want to error message in my php pages. But I want to change the message corresponding to the error. This is my code. But it is not working. Please tell me where I goes wrong?
This is the error message depen on the error.

$MESSAGE = '<p class="errMsg" align="Center">Invalid Username or Password</p>'

And this error message display on the correct place like this

if(isset($MESSAGE)){ echo "$MESSAGE"; }

[Dormilich]

from these two lines it is impossible to say where it went wrong.

[ghjk]

ok. This is my full code.

[LEFT]
if(isset($_GET['S'])==1){
	$make=$_GET['make'];
	getVehicleModel($make);
	return;
	}
//------if user not loggerd in then set value as null------------------------------------------------
if(!isset($_SESSION['UserName_Ses']))
{ 
$_SESSION['UserName_Ses'] = "";
}
if(!isset($_SESSION['UserType_Ses'])) {
$_SESSION['UserType_Ses'] = "";
}
if(!isset($_SESSION['login'])) {
$_SESSION['login'] = "";
}
// ----------- if user click login button--------------------------------------------------------------
if(isset($_POST['login'])){

	$username=$_POST['userusername']; 
	$password=$_POST['userpassword'];
	if(($username=='')||($password==''))
	$MESSAGE = '<p class= "errormsg">Please enter all fields</p>';
	
	if((!$username)||(!$password))
	
		$MESSAGE = '<p class= "errormsg">Please enter all fields</p>';
		
	
		$password_encrpt = md5(substr($password, 1, -1));
		
		$sql = "SELECT * FROM user WHERE  userName='$username' AND userPassword ='$password_encrpt'"; 
		$result=mysql_query($sql)or die(mysql_error());
//----------this is use to retriew values---------------------------------------------------------------		
		$row = mysql_fetch_array($result) or die(mysql_error());
//-----------if login success then get user typr to session and go to the login page--------------------		
		if(mysql_num_rows($result) == 1){
			$_SESSION['UserType_Ses'] =$row['userType'];
			$_SESSION['UserNamee_Ses'] =$row['userName'];
			$_SESSION['login'] = "True";
			header('Location: index.php');
				
			
		}
		else {
			$MESSAGE = '<p class="errMsg" align="Center">Invalid Username or Password</p>';
			echo "error";
		}

}


?>
<table width="566" height="100%" border="0" align="center" cellpadding="0" cellspacing="0"  class="mainTblBorder">
  <!-------------------hedder table-------------------------------------------------->
  <tr>
    <td><?php require_once 'hedder.php'; ?>
    </td>
  </tr>
  <!--------------menu-------------------------------------------------------------->
  <tr>
    <td><?php require_once 'menue.php'; ?>
    </td>
  </tr>
  <!--------------body-------------------------------------------------------------->
  <td>
	  <table width="520" id="maintable">
		<tr>
			<td width="234">
			<?php if ($_SESSION['login'] != 'True')  {
			login();
			}?>
			
				<?php if ($_SESSION['login'] == 'True')  {
				//header('Location: stockAdmin.php');	
				
			
			}?>
		    </td><td>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td>
		  <td><br /><table width="372" border="0" cellpadding="0" cellspacing="0" class="subTableBorder" id="stocksearch">
            <tr>
              <td colspan="2" align="center"><img src="http://bytes.com/images/subBannerStockSearch.jpg"  /></td>
            </tr>
			<tr height="20"><?php  if(isset($MESSAGE)){ echo "$MESSAGE"; }?></tr>
            <tr>
              <td  height="20">&nbsp;Make&nbsp;&nbsp;:</td>
              <td ><?php  

					?>
			 </td>
            </tr>
            <tr>
              <td  height="30">&nbsp;Model&nbsp;:</td>
              <td><div id='txtHint'><?
					getVehicleModel($make);
					?></div>
			</td>
            </tr>
            <tr>
              <td  height="30">&nbsp;Year&nbsp;&nbsp;&nbsp;:</td>
              <td><? getVehicleYear(); ?></td>
            </tr>
            <tr>
              <td  height="30">&nbsp;Fuel&nbsp;&nbsp;&nbsp;&nbsp;:</td>
              <td><? getVehicleFuel();?> </td>
            </tr>
			<tr><td colspan="2"align="right">
			<button type="submit" name="login" style="border: 0; background: transparent; cursor: pointer;">
   			<img src="http://bytes.com/images/searchBtn.jpg"  />
			</button>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br /><br /></td></td>
			</tr>
          </table></td>
		</tr>
	  </table>
    </td>
 	 </tr>
	 
	  	<tr>
    		<td>
			<table align="center">
				<tr>
					<td>
				
					<?  getNewArrivals(); ?>
					</td>
				</tr>
			</table>
    		</td>
  		</tr>
	 
	  	<tr>
    		<td><?php require_once 'footer.php'; ?>
    	</td>
  </tr>
</table>

<?php
function login(){
?>				<form action="index.php" method="post">
				<table width="98%" height="179" border="0" cellpadding="0"  cellspacing="0" class="subTableBorder" id="customerLogin">
					<tr valign="top">
					  <td colspan="2" width="100%"><img src="http://bytes.com/images/subBannerCustomerLogin.jpg" /></td>
					</tr>
					<tr><td height="24" colspan="2">Please login to access members privilages</td>
					</tr>
					<tr valign="top">
					  <td  height="30">Login ID&nbsp;&nbsp;:</td>
					  <td><input name="userusername" type="text" id="userid" /></td>
					</tr>
					<tr>
					  <td>Password:</td>
					  <td><input name="userpassword" type="password" id="userpassword" /></td>
					</tr>
					<tr>
					  <td height="53" colspan="2" align="right" ><br />
				     <!-- <input type="submit" name="login"  value="Submit" class="btn" height="30"  width="50"/>-->
					<button type="submit" name="login" style="border: 0; background: transparent; cursor: pointer;">
   					 <img src="http://bytes.com/images/loginBtn.jpg"  />
					</button>
					  </td>
				  </tr>
				  <tr>
				  	<td align="center">
					  <strong>Not yet registered?</strong> </td>
					<td class="signUpTxt"><a href="signUp.php" style="text-decoration:none">Sign-up Now!</a></td>
				  </tr>
				  <tr><td colspan="2" align="center"><br />Forgot Username? </td></tr>
				 
  </table>	
</form> 
<?php }
?>
<!-------------------------end stock search---------------------------------------- -->
	
</body>
</html>[/LEFT]